Lean burn engines, such as diesel engines, are generally operated at a higher than stoichiometric air to fuel mass ratio to improve fuel combustion efficiency and to provide good fuel economy. The exhaust emission produced by such engines generally contains at least four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). Emissions standards for lean burn engines, whether stationary or mobile (e.g. vehicular engines), are being progressively tightened. There is a need to provide improved emissions control devices that are able to meet these standards. One such emissions control device is a nitrogen oxides (NOx) storage catalyst (NSC). NSCs are also referred to in the art as NOx adsorber catalysts (NACs), lean NOx traps (LNTs), deNOx traps (DNTs) or NOx storage/reduction (NSR) catalysts.
During normal operation, a lean burn engine produces an exhaust emission having a “lean” composition. The NSC is used to store or trap the nitrogen oxides (NOx) that are present. Nitrogen dioxide (NO2) present in the exhaust emission is typically adsorbed by a NOx storage component of the NSC, which stores the NO2 through the formation of an inorganic nitrate. The nitric oxide (NO) component of NOx in the exhaust emission is usually catalytically oxidised by the NSC to nitrogen dioxide (NO2). The additional NO2 that is formed may then be stored by the NOx storage component of the NSC.
To release the NOx from the NOx storage component, such as when the NOx storage component is about to reach its storage capacity, the lean burn engine may be run under rich conditions to produce an exhaust emission having a “rich” composition. Under these conditions, the inorganic nitrates of the NOx storage component decompose and form mainly nitrogen dioxide (NO2) and some nitric oxide (NO). The NSC may contain a platinum group metal component that is able to catalytically reduce the released NOx to N2 or NH3 with hydrocarbons (HCs), carbon monoxide (CO) or hydrogen (H2) present in the exhaust emission.